1. Field of the Invention
The present invention relates to a retransmission control technique in a mobile communication system and, more particularly, to a mobile communication system which uses broadcast communication or multicast communication, a mobile terminal, base station, and radio network controller which constitute part of the system, a retransmission control method used therein, and a program therefor.
2. Description of the Related Art
In a mobile communication system such as a cellular mobile communication system, when there are many mobile terminals that receive the same service within a single cell, a base station or mobile terminal performs data transmission using a common channel of a radio network. Use of such a common channel contributes to effective utilization of radio resources. For example, Japanese Patent Laid-Open No. 2004-320481 describes use of this common channel.
However, if a network provider is to provide a service by broadcast communication or multicast communication using a common channel, it is difficult for the network provider to provide the service to all mobile terminals with almost equal reception quality. A mobile terminal at the edge of a cell is particularly likely to suffer interference or a reduction in power which occurs during data transmission and often receives data with lower reception quality than other terminals.
When using a common channel, a base station cannot individually control mobile terminals with ease. For this reason, it is more difficult to avoid a reduction in data quality in data communication using a common channel than that using individual channels.
To cope with this, there can be considered combination of the common channel technique and a selective combining technique (or a soft combining technique). In this combined technique, a plurality of cells in a radio network are integrated into one or more groups in advance. All base stations (or Node-Bs) that belong to a single cell group simultaneously transmit identical data. In this case, a mobile terminal at the edge of a cell can also simultaneously receive identical data transmitted from an adjacent cell. Accordingly, the combined technique moderates a reduction in reception data quality.
In the technique, when a terminal which belongs to a cell group detects an error in received data, it transmits a negative acknowledgment (NACK) to a base station. In response to the negative acknowledgment, all base stations that belong to the cell group perform data retransmission. More specifically, even when only terminals that belong to some cells within the cell group detect an error in received data, all the base stations within the cell group retransmit the data which the some terminals have failed to properly receive. This data retransmission unnecessarily increases the traffic of cells to which only terminals having properly received the data belong.
As described above, when using a common channel at the time of broadcast transmission or multicast transmission, unnecessary data retransmission may prevent efficient use of network resources.
Under the circumstances, there is a need for a technique for improving the reception quality by using a selective combining technique (or a soft combining technique) while maintaining the advantage in using a common channel, i.e., efficient use of network resources.